This invention relates to a well screen such as an oil well screen and, more particularly, to a method of manufacturing a selective isolation screen in which, in a wire wrapped screen, seal means for isolating and sealing annulus defined by spacer members disposed on the outer surface of a base member such as a slitted pipe is provided at selected locations in the longitudinal direction.
A screen which is used most frequently as a deep well screen such as an oil well screen has a perforated pipe formed with multitudes of circular openings or slits as its base member, a plurality of spacer rods extending in the longitudinal direction arranged at intervals in the circumferential direction on the outer surface of the base member and a wire wound spirally on the outer periphery of the spacer rods with a predetermined gap. By the provision of the spacer rods, annulus defined by the spacer members and extending in the longitudinal direction is formed between the inside of the wire and the outer surface of the base member over the entire periphery of the base member.
This annulus defined between the inside of the wire and the outer surface of the base member is indispensable for securing a sufficient flow rate of fluid by enabling fluid flowing from the gap of the wire to flow uniformly to openings of the base member. The provision of the annulus, however, causes the following problems.
In a case where plugging has occurred in a screen, an operation is made for dissolving or removing materials which have caused plugging by locally injecting liquid such as hydrochloric acid, diesel oil, light oil or surfectant under pressure in the radial direction from inside of the base member. This liquid, however, is dispersed vertically through the annulus and, as a result, a sufficient amount of liquid is not injected concentrically to the location at which the plugging has occurred and dissolving or removal of the plugging materials is not achieved to an expected degree.
In a screen of a type in which gravel is filled in a wellbore about the screen for preventing entering of sand into the screen by the filled gravel, fluid is injected towards the gravel in the radial direction from the inside of the base member when gravel is filled in the wellbore to force the gravel down by the pressure of the fluid injected. In this case also, the fluid injected is dispersed vertically through the annulus so that gravel does not move down as expected with resulting loss of uniformity in the distribution of gravel in the wellbore and reduction in the effect of preventing entering of sand into the screen.
For overcoming the above described problems, U.S. Pat. No. 4,771,829 proposes a selective isolation screen in which seal means for isolating and sealing annulus extending in the longitudinal direction defined by spacer rods of the above described type of screen are provided at selected locations in the longitudinal direction. In this screen, as shown in FIG. 6, a plurality of spacer rods b are arranged on the outer surface of a base member a consisting of a perforated pipe, a wire c is spirally wound on the outside of the spacer rods b and cylindrical sleeves e having the same radial length as the radial length of the spacer rods b are disposed inside of the wire c at selected locations at intervals in the longitudinal direction and fixed to the spacer rods b. By disposing these sleeves e which constitute the seal means at selected locations in the longitudinal direction, the annulus defined by the spacer rods and extending in the longitudinal direction is isolated and sealed in the longitudinal direction. Accordingly, in taking a step for removing plugging or filling gravel in a wellbore, fluid injected from the inside of the base member in a screen section where plugging has occurred or gravel to be forced down exists is injected radially towards the wellbore through the gap of the wire c, for the vertical flow of the fluid in the annulus is restricted by the sleeves e provided at the upper and lower locations of the screen section where the fluid is injected.
As described above, the selective isolation screen is useful for achieving effectively removal of plugging and flow down of gravel but the prior art selective isolation screen has the following problem.
In the prior art selective isolation screen, as shown in FIG. 6, the wire c is wound over the sleeves e. The wire c is normally spot-welded to the spacer rods b. Since, as is well known, spot-welding is effected by concentrating electric current at a relatively small area to heat it locally, it is extremely difficult to spot-weld the wire c to the surface of the sleeves e which have a relatively broad area as shown in FIG. 6.
Further, in the prior art selective isolation screen, the sleeves e are covered by the wire c and the gap of the wire c is normally of a very small value of about 0.3 mm and, accordingly, the sleeves e are not seen from the outside and locations and state of mounting of the sleeves e cannot be visually checked. This structure is inconvenient in that the screen has to be used without checking the state of the sleeves even if some there is defect in mounting of a sleeve.
It is, therefore, an object of the invention to overcome the above described problem of the prior art selective isolation screen and provide a method of manufacturing a selective isolation screen in which difficulty in welding work can be avoided and location and state of mounting of the sleeves can be readily checked visually.